Generally, a mobile communication system provides a handover function such that a call can be continuously maintained though a mobile terminal departs from a service area (A) of a current base station and then advances into a service area (B) of an adjacent base station as shown in FIG. 1, thereby ensuring mobility.
Referring to FIG. 2, for conducting the handover function in the WCDMA network, when a pilot signal strength of a B sub-cell is detected to exceed a threshold value (add), for example, a mobile terminal (MS) 10 transmits MR (Measurement Report to a radio network controller (NEC) 11, and the RNC 11 analyzes MR and then instructs channel allocation to a corresponding base station if B sub-cell information acting as a target sub-cell exists in a neighbor list, and then transmits an active set update message to the mobile terminal 10 after the channel allocation is conducted. After that, the mobile terminal 10 transmits an active set update completion message to the RNC 11, thereby completing the handover processing.
In an asynchronous WCDMA network, pilots are managed in groups classified into an active set, a monitored set, and a detected set. Here, the active set is a group of PSC (Primary Scrambling Code) that are pilot offsets associated with a traffic channel currently connected to the mobile terminal. The monitored set is a group of pilot offsets existing in a cell info list but not belonging to the active set. The detected set is a group of pilots not belonging to the active set and the monitored set.
For the mobile terminal to successfully conduct handover, information of a target sub-cell should be exist in a neighbor list corresponding to a source sub-cell. Thus, it is very important to correctly make and update the neighbor list.
FIG. 3 shows a handover failure at points t and t at which handover is conducted incase the information of a target sub-cell does not exist in the neighbor list. As shown in FIG. 3, when a pilot signal strength of the B sub-cell is detected to exceed a threshold value (add), the mobile terminal 10 transmits MR to the RNC 11, and the RNC 11 analyzes MR and then generates a call fault informing the handover failure if it is checked that a target sub-cell does not exist in the neighbor list.
The neighbor list is generally made and updated based on manual operation of an operator, which thus takes much data analyzing time, gives possibility of subjective recording or errors, and also takes much time to verify a prepared neighbor list. Thus, there is a need for a scheme capable of automatically optimizing a neighbor list.
As a related technique, Korean Laid-open Patent Publication No. 2004-48038, entitled “Method for automatically updating a neighbor list in a CMDA communication network”, discloses increasing a handoff success rate by adding a handoff (handover) failed PN to a neighbor list using a neighbor PN (Pseudo Noise) statistical list, or by adjusting a neighbor list extract priority upward according to a handover trial frequency for the handoff-failed PN. However, this technique executes the neighbor list updating process per each base station controller (BSC), so it cannot receive statistical information on a PN not related to the corresponding BSC and thus it cannot update the information.
Korean Laid-open Patent Publication No. 2005-24643, entitled “Method and apparatus for generating a handover neighbor list in a cellular mobile communication system”, discloses determining priorities of subject base stations according to an order of handover occurrence frequencies and then generating a neighbor list according to the order. However, this technique has a limit in analyzing works since it uses handover statistical information per each base station, so it is difficult to give a highly reliable list. In addition, since it is difficult to get accurate location information of a base station not existing in a neighbor list but substantially included in statistics, it hardly calculates an accurate neighbor list, and also it cannot analyze a call fault generated when there is a handover request for a sub-cell in a remaining set.